The Changing of the Leaves

I realise it is well into winter for many four-season countries around the globe. Soon it will give way to spring, then summer, then fall will come like it always does. And you’d once again be witnessing the leaves turn gold and fall, all in preparation for the winter that would again come. 

Everything changes, it always does. Yet nature never does. Amazing, isn’t it? Every year, at a certain point in time, the nights start growing longer. Every year, the leaves on deciduous trees start undergoing microscopic processes resulting in brightly coloured foliage and leaf-strewn paths. Every child’s fascination, remember?

Rewind a bit. Why does fall exist?

It’s a cycle. The life cycle of a leaf ends with abscission, when it wilts and drops from the tree, no longer connected to its source of life. Not very pleasant, but what is strength without weakness? 

It’s not just deciduous trees that arm thermselves for the battle against harsh winters every year. Evergreens, for instance, like pines heavily coat their needle-shaped leaves with wax. Deciduous trees, however, have broad and flat leaves that are vulnerable to freezing temperatures due to the watery fluid in their cells: if this solidifies, the solid ice would end up taking up more space and rupture the cells. Thus the discarding of leaves, so as not to drain the tree’s limited food supply for the winter.

Changing colours: chlorophyll, carotenoids and anthocyanins 

The autumn palette is arguably one of nature’s most glorious. If you want to understand the mechanisms behind this, you need to be familiar with these three pigments found in the leaf:

• Chlorophyll is what gives leaves its green colour during the growing season; it is responsible for the production of sugars, using energy from sunlight

• Carotenoids—their work you can see in pumpkins, carrots, and most crops yellow or orange

• Anthocyanins are water-soluble pigments whose colours vary with pH showing red, blue, or black. 

In the summer, chlorophyll would continually be made and broken down but when the nights turn longer and photosynthesis can no longer take place at the same rate, the green pigment is produced less and less: eventually, chlorophyll production stops and can no longer be found. Now the orange/brown/gold colour of the carotenoids and anthocyanins are left uncontested, exposed. 

Anthocyanins are for the most part only produced in the autumn: they are produced in response to bright light and excess plant sugars within leaf cells. This is a result of the veins in the leaves clogging up in cold temperatures, which gives the manufactured sugars in the leaves nowhere to go. 

Following this explanation, therefore, it can be concluded that if you had deciduous trees growing somewhere where the days were sunny but the nights were sharp and chilly, the rate of photosynthesis in the leaves would be higher during the day leading to a greater concentration of sugars—and veins would be able to close off in the night. While carotenoids are always present, these conditions would especially drive the production of anthocyanin pigments, giving us crimson or even purple leaves. Of course, temperature does not alone influence such happenings. Moisture plays a role too: the damper the soil (to a certain extent), the more intense the colour.

So from now on, whenever you see a tree or so much as a fallen leaf, remember the complexity of the mechanisms that lead to such radiant displays of fervor and meaning. Don’t just “stop and smell the roses”: pick it up, study it, and look into what lies inside.